Worried about the flu season? Here’s the story behind the figures

Influenza can be a serious and even life-threatening illness, but most infections are mild and self-limiting. HI TRICIA! 王 圣 捷/Flickr, CC BY-NC-SA

Today’s reports suggesting a particularly severe flu season could easily be overstating the case. The figures, released by Influenza Specialist Group say there have been more than 20,000 cases of flu nationally so far this year, double last year’s number.

The release highlights the fundamental problem with trying to get a picture of the flu’s impact. This 20,979 figure comes from labs confirming the presence of the virus in samples sent to them by doctors working in hospitals or in the community.

This is known as notification data and placing too much reliance on these data can lead to an exaggeration of seasonal severity because testing has become more widespread and frequent.

Testing increased after the 2009 swine flu pandemic, with increased flu awareness among doctors and more labs performing tests. Increased testing has meant increased notifications, but not necessarily increased disease or severity.

This graph of nationally notified cases puts 2014 in perspective:

National Notifiable Diseases Database

While influenza can be a serious and even life-threatening illness, most infections are mild and self-limiting, and the 2014 season is unlikely to be any different.

There’s certainly been a rise in cases this year but we don’t yet know how high it will go. As the table above shows, it’s now a bit higher than 2013, but we don’t know whether the 2014 season is just starting to rise or has already peaked.

The 2013 season was late and mild, so it’s not really surprising that there are more cases this year than at the corresponding time last year.

The 2014 flu season so far

This is what we know about this year’s flu season: outbreaks have been reported in aged-care residential facilities, even where staff and resident vaccination coverage have been high. And in the community, younger people are seeing doctors with influenza-like illnesses.

Sounds pretty bad, right? It is but it’s also a little more complex than is usually acknowledged. And that’s often because acknowledging complexity can make public health messaging and vaccine marketing more difficult. So this is what you need to know.

First, influenza is a common infection that has a very wide clinical spectrum; infection can occur without any symptoms, without fever, or with a few mild respiratory symptoms. A recent five-year study from England, for instance, found less than half of all laboratory confirmed flu infections were associated with symptoms.

But it may also be associated with serious respiratory and other symptoms requiring intensive care admission.

A recent UK study found less than half of all laboratory confirmed flu infections were associated with symptoms. Lotus Carroll/Flickr, CC BY-NC-SA

So, most infections are mild and resolve on their own but we obviously want to prevent serious infections. Vaccination is the best prevention measure but it is by no means perfect.

The flu vaccine

The important thing here is that vaccines aren’t all created equal. Rubella and measles vaccines are about 90% effective at preventing disease but the type of flu vaccines we have in Australia (inactivated vaccines) are only about 50% to 70% effective.

Some years, they don’t even make this grade: an estimate for the 2012 vaccine in Australia suggested it was only 23% effective.

What this means is that people should not expect to be protected from influenza just because they have been vaccinated. What they should expect is a decrease in their risk of infection. Most years that overall risk should be halved.

Another thing to keep in mind is that when we estimate the effectiveness of the influenza vaccine, we’re only estimating protection against disease we can identify.

We don’t know, for instance, whether the vaccine has a beneficial effect on mild disease. And because the circulating strains and the vaccine usually change every year, the effectiveness of the vaccine varies by year.

It gets more complicated

Although we can all get flu from any type of the influenza virus, people of different ages are likely to suffer adverse outcomes from different types.

The type of flu vaccines we have in Australia (inactivated vaccines) are only about 50% to 70% effective. Lance McCord/Flickr, CC BY-NC-SA

The virus known as swine flu, which is the influenza subtype H1N1, for instance, is more likely to infect younger people, but the consequences of infection are still worse for older people if they’re infected.

But the aged-care facility outbreaks reported this season have mostly been caused by a different subtype of influenza virus, H3N2. This is the subtype generally associated with more severe influenza seasons. Although H1N1 dominated the season early, we’re now seeing the emergence of H3N2 viruses.

The influenza vaccines available in Australia are made of the components of three flu viruses – H1N1, H3N2 and B. As I said above, they’re changed every year in an attempt to provide protection against the right strain of the virus.

But here’s the rub. For a number of years now, inactivated influenza vaccines have not worked as well against H3 as they have against H1.

In older people, in particular, effectiveness against H3 has been very poor; that’s why we’re seeing outbreaks in aged-care facilities despite high vaccine coverage in those places.

What this means for you

So, let’s put this all into perspective.

Influenza is common. It presents with a wide clinical spectrum, from infection without any symptoms to infection leading to death. You might be bedridden or you might not even notice you’ve got it.

You could end up in bed when you have the flu or you could not even notice you’ve got it, or fall somewhere in between. William Brawley/Flickr, CC BY

Despite the usual messages, influenza symptoms can resemble the common cold. But unlike the common cold, flu can kill some people.

Death is more common in older adults with other medical problems, but also occurs – rarely – in otherwise healthy children. Serious outcomes, like hospital admission, are uncommon for healthy adults and death in this age group is exceedingly rare.

Although vaccination remains the best option for prevention, influenza vaccines don’t offer the sort of protection we’ve come to expect from the best childhood vaccines.

Finally, we are in the midst of this year’s influenza season. Over the next few weeks, many of us will be infected without even knowing it. And a small number of us will become seriously ill. Those who are at serious risk of an adverse outcome from infection should be vaccinated but realise that vaccination will not guarantee protection against influenza.